1. A method of treating a disease or disorder associated with TrkB activity or expression, or for ameliorating at least one symptom associated with the disease or disorder associated with TrkB activity or expression which is characterized by nerve cell injury or nerve cell degeneration in a subject in need thereof, comprising administering a therapeutically effective amount of an isolated antibody or antigen-binding fragment thereof that binds specifically to tropomyosin receptor kinase B (TrkB), wherein the antibody or antigen-binding fragment thereof comprises a set of six complementarity determining regions (HCDR1-HCDR2-HCDR3-LCDR1-LCDR2-LCDR3) comprising an HCDR1 comprising the amino acid sequence of SEQ ID NO:4, an HCDR2 comprising the amino acid sequence of SEQ ID NO:6, an HCDR3 comprising the amino acid sequence of SEQ ID NO:8, an LCDR1 comprising the amino acid sequence of SEQ ID NO:12, an LCDR2 comprising the amino acid sequence of SEQ ID NO:14, and an LCDR3 comprising the amino acid sequence of SEQ ID NO:16 to the subject.